Introduction to Binary Stars

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Introduction to Binary Stars

• Steve B. Howell
• NOAO

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Binary Stars ??
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Binary Stars ??

• 0 1

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Binary Stars -
Binary stars are pairs of stars orbiting each
other connected by their mutual gravitational
interaction. 50 of stars are binaries
100 O/B 75 M Obs. biases
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Binary Stars -

• Binary Stars may have orbital periods from
  seconds to hours to thousands of years
• They may contain stars of all kinds, sizes, and
  masses
• They may have circular or elliptical orbits
• They are believed to have formed together - same
  age (not capture)
• Close binaries affect each others evolution
• Long period binaries essentially act as single
  stars

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Binary Stars - Types

• Binary stars come in a few types differentiated
  by their orbital period and/or observational
  behavior. For example
• Visual double (actually see both components -
  LONG period). Do not confuse with optical double
• Astrometric binaries - see the wobble on the sky

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Binary Stars - Types
An astrometric and visual binary Sirius AB --
an A star (A) and a white dwarf (B) Has a 50
year orbit. 8.6 light years from Earth
Chandra Image
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Binary Stars - Types

• or
• Eclipsing binaries - one/two eclipses per orbit
• Spectrum binaries - spectral properties change
• Spectroscopic binaries - radial velocity motions
  (short periods easier to find)

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Binary Stars - Types
Schematic and real eclipsing binary light curves
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Binary Stars - Types
Spectral variations over time due to the binary
nature of this star
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Binary Stars - Types

• Or differentiated by other criteria-
• Close binaries w/ stellar distortions or mass
  transfer
• Intrinsic brightness changes due to stellar
  variability
• Binaries with components of very different
  properties color (WDRD), mass (Exo-Planet),
  temperature
• Other

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Binary Stars - Types
Binaries can be close (short orbital period) and
show light variations due to Ellipsoidal shape
and/or tidal interaction Interactions such as
mass exchange / transfer If the smaller (but more
massive) star in an IB is a white dwarf, the
binary is called a cataclysmic variable
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Binary Stars - Types

• All combinations of the previous properties are
  possible
• Many are unknown as they simply have not been
  seen to vary (line of sight / observed)
• Binary Stars are the fundamental way in which we
  understand stars (and star systems) their
  formation, evolution death.
• Remember --Three out of every two stars are in a
  binary system

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Binary Stars - What can we learn?

• Eclipsing binaries are the best as the light
  curve can give stellar masses, radii,
  temperatures, age estimates

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Binary Stars - What can we learn?

• Eclipsing (or not) Radial velocity curve can give
  masses (or mass estimate)

RV amplitudes give K1 K2
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Binary Stars - What can we learn?

• For a circular orbit

Measure semi-major axis, a, from projected orbit
the distance. Relative positions about the
center give M1/M2 a2/a1 K2/K1
Keplers Law
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Mass vs. radius and luminosity
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The Oddest Eclipsing Binary?
1987 Epsilon Aurigae model from web site --gt
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The Oddest Eclipsing Binary Life??
Elemarkhorsaurus from the Epsilon Aurigae Binary
Star System Caption Ashraf, do you want to meet
my new pet? Look, he is behind you and friendly.
He loves the climate and his new home in the Eps
Aur binary system. (From flickr)